1. Field of the Invention
The present invention relates to a ringing signal generator for a telephone central office which is capable of producing a time multiplexed ringing series.
2. Description of the Prior Art
A multifrequency (MF) ringing system is one which typically includes a ringing signal generator which produces a time multiplexed series of four different ringing signals having mutually different frequencies and corresponding amplitudes. In known arrangements, four different ringers in subscriber telephone instruments are connected in parallel across the tip and ring signal lines. Each of these ringers is tuned to one of the four ringing signal frequencies in the series of ringing signals. When it is desired to ring a particular one of the ringers, the ring signal with the frequency to which the selected ringer is tuned, and having the corresponding amplitude is placed on the tip and ring signal lines.
In the central office, a series ring signal generator repetitively produces a time multiplexed series of the four different ring signals. The appropriate ring signal from such a ring signal generator is supplied to the ringer in the selected subscriber telephone instrument by coupling the output of the ring signal generator to the tip and ring signal lines during the time interval when the appropriate ring signal is being produced by the series ring signal generator.
Series ring signal generators are known. U.S. Pat. No. 4,349,703, issued Sept. 14, 1982 to Chea, Jr. describes a ring signal generator which is capable of producing one of four possible ring signals having different frequencies. FIG. 1 of this patent illustrates four separate ringing frequency generators 20-26. These are coupled to tip T and ring R lines (14 and 16 ) through ringing circuits 10 and 12, respectively. "... [T]he energizing and deenergizing of the ring circuitry is programmably controlled to provide for either hardware or software generation of ringing cadence ...." (See column 2 lines 15 to 18.)
Apparatus according to this patent requires four ringing signal generators which must be maintained to keep the frequencies and amplitudes within acceptable limits. In addition, the separate ringing signal generators are not phase aligned so that switching from one to another to form the time-multiplexed sequence introduces impulse noise into the ringing signal sequence. A ringing signal generator which comprises a single unit is desirable. In addition, a ringing signal generator in which each of the component ringing signals in the time multiplexed sequence begins and ends on a zero crossing is also desirable. In addition, a ringing signal generator which is capable of changing the individual components in the time multiplexed sequence, or is capable of selectably producing one of a plurality of different ringing signal sequences is also desirable.